Can I Use Bitumen Paint to Waterproof a Below-Ground Foundation Wall?

Bitumen paint is a legitimate and widely used option for waterproofing below-ground foundation walls in New Zealand — but it's not always the best or most durable solution. Whether it's right for your situation depends on your soil conditions, water table, wall material, and how much moisture pressure the wall faces. Here's what you need to know before you reach for the brush.

What Is Bitumen Paint and How Does It Work?

Bitumen paint (sometimes called bituminous paint or black bitumen coating) is a solvent- or water-based product made from bitumen — the same base material used in roofing felt and asphalt. When applied to a concrete or masonry surface, it cures to form a flexible, water-resistant membrane that helps prevent moisture from penetrating through the wall.

It's relatively affordable, easy to apply with a brush or roller, and widely available at stores like Bunnings and Mitre 10. For light-duty applications — such as protecting a concrete block foundation wall from damp soil contact — it can perform well.

When Bitumen Paint Is a Reasonable Choice

Bitumen paint is generally suitable when:

• The wall faces low to moderate soil moisture rather than active groundwater pressure
• You're dealing with damp-proofing (keeping out moisture vapour and light seepage) rather than full waterproofing (resisting hydrostatic water pressure)
• The substrate is concrete, masonry block, or brick in sound condition
• The area is not subject to significant ground movement or tree root activity
• It's used as part of a drainage system (e.g., alongside a subsoil drain and gravel backfill) rather than as the sole line of defence

For a typical NZ home with a concrete perimeter foundation wall in reasonably free-draining soil, two or three coats of bitumen paint applied to the exterior face before backfilling is a common and accepted practice.

The Limitations You Need to Understand

Bitumen paint is not a heavy-duty waterproofing membrane, and it has real weaknesses:

• Hydrostatic pressure: If your site has a high water table or water pools against the wall after rain, bitumen paint alone will not hold. You need a purpose-built waterproofing membrane system.
• UV degradation: Bitumen breaks down quickly when exposed to sunlight, so it must be covered by backfill or another protective layer.
• Cracking: It can crack over time, especially if the wall moves or settles. Flexible polymer-modified membranes handle movement better.
• Thin film: A brush-applied coat is much thinner than a sheet membrane or a trowel-applied cementitious system — coverage and consistency matter enormously.
• Surface prep is critical: Any contamination, loose material, or moisture on the surface at time of application will compromise adhesion and performance.

Better Alternatives for Higher-Risk Situations

If your site has significant water ingress risk, consider these alternatives or use them in combination with bitumen paint:

For guidance on what the NZ Building Code requires for below-ground moisture control, refer to Clause E2 (External Moisture) and check MBIE Building Performance for the relevant Acceptable Solutions.

How to Apply Bitumen Paint Correctly

If bitumen paint is appropriate for your project, follow these steps:

1. Clean the surface thoroughly — remove dirt, dust, efflorescence, and any loose material. Allow the wall to dry if using solvent-based product (some water-based products can be applied to damp surfaces — check the manufacturer's datasheet).
2. Fill cracks and holes with a suitable mortar or filler and allow to cure.
3. Apply a primer if recommended by the manufacturer.
4. Apply two to three coats, allowing each coat to dry fully before the next. Pay extra attention to joints, corners, and penetrations.
5. Protect the coating before backfilling — use a drainage board or geotextile to prevent damage from stones and backfill material.
6. Install a subsoil drain at the base of the wall to carry groundwater away. This is arguably as important as the coating itself.

Always read and follow the specific product's technical data sheet. Brands such as Wattyl and others stock bituminous products with detailed application guides.

Does This Work Require a Building Consent?

Waterproofing an existing foundation wall as maintenance or repair is generally exempt from building consent under Schedule 1 of the Building Act — but this depends on the scope of work. If you're doing significant excavation, structural work, or altering drainage, consent may be required. Always check with your local council before starting. The MBIE Building Performance website has useful guidance on consent exemptions.

BRANZ also publishes research and good practice guides on below-ground moisture management that are worth reading before you start.

When to Call a Professional

You should engage a qualified waterproofing tradesperson if:

• There is active water ingress or a high water table on your site
• The foundation wall shows structural cracking or movement
• You're unsure whether bitumen paint alone will be adequate
• The project involves significant excavation close to the foundation
• You want the work to carry a warranty or be covered by your home insurance

A professional waterproofer will assess your specific site conditions and recommend the right system. You can find qualified local waterproofing tradespeople on tradedirectory.co.nz.

Sources & Further Reading

• MBIE Building Performance – Building Code Clause E2 External Moisture
• BRANZ – Moisture and Below-Ground Construction Guidance
• Sika NZ – Waterproofing Products and Technical Data
• Ardex NZ – Waterproofing Systems
• Bunnings NZ – Bitumen and Waterproofing Products
• Mitre 10 NZ – Waterproofing and Damp-Proofing